DISTILLATION OF STUMPWOOD. 45 



a larger force are required in operating than for the uncontrolled 

 process. For this reason it is rarely used for distilling resinous 

 wood. 



STEAM DISTILLATION PROCESS. 



The steam distillation process requires that the wood to be ex- 

 tracted shall be finely divided by chipping or shredding before treat- 

 ment ; the finer the chips, the more rapid and complete the extraction. 

 For this reason the steam process has been installed by several saw- 

 mills for the recovery of turpentine from sawdust. The best results 

 are not obtained with all dust, however, as it packs so tightly that 

 the steam is kept from penetrating throughout the entire mass to be 

 extracted. Chips of a size passing an inch and retained by a quarter- 

 inch screen are desirable, and a limited amount of sawdust can be 

 mixed with such chips. 



Few plants, other than lumber mills where the production of wood 

 turpentine and pine oil is only a side issue, have continued to operate 

 on the steam process alone, and have invariably closed when turpen- 

 tine sold at less than 50 cents a gallon. The turpentine produced by 

 this method is of high quality, approaching that made by the regular 

 distillation from gum. The practicability of maintaining a steam 

 distillation plant depends entirely on market conditions ; if the price 

 of turpentine is sufficiently high the steam method will be a paying 

 proposition. The steam distillation outfit is now usually installed in 

 conjunction with a solvent plant that can extract the residual wood 

 chips for the recovery of rosin and certain of the heavier pine oils. 



SOLVENT EXTRACTION PROCESS. 



In the solvent process also the wood must be finely divided. This 

 process is one where the wood is extracted in large, tight digesters 

 at a relatively high temperature by means of suitable volatile sol- 

 vents, the choice of which is determined mainly by price. Gasoline, 

 coal tar, naphtha, or turpentine can be used, gasoline being the one 

 in common use. When the solvent is added in the beginning of the 

 operation, that is, with no previous steam distillation, all of the solu- 

 ble pine products are removed altogether, and the resulting mixture 

 is fractionated to recover the naphtha or other solvent and to sepa- 

 rate the turpentine and pine oils from the rosin. The rosin obtained 

 in this way is not so free from tackiness as pure gum rosin, and has 

 a rather darker color, but is quite clear when properly made. Fur- 

 thermore, it is very difficult to remove the solvent completely from 

 the turpentine. It has been found advantageous, therefore, to com- 

 bine the steam and solvent processes, the only objection' to this being 

 that the steam leaves the chips in a moist condition, in which state 

 the extraction does not take place as readily as if they were abso- 

 lutely dry. 



